1. Field of the Invention
This invention relates to electrographic development apparatus for controlling the concentration of electroscopic toner particles in an electrographic developer mixture. More specifically, this invention relates to improvements in toner concentration monitoring apparatus of the type which senses toner concentration by sensing variations in the dielectric constant of the developer mixture.
2. Description of the Prior Art
In the electrographic reproduction process, the surface of a dielectric member, generally comprising a layer of photoconductive material disposed on a conductive backing, is given a uniform electrostatic charge and is then imagewise exposed to a pattern corresponding to the indicia on a document or the like being reproduced. Such exposure serves to selectively dissipate the uniform charge on the surface, leaving behind a latent electrostatic image which can then be developed by contacting it with an electrographic developer mixture.
In general, electrographic developer mixtures comprise suitably pigmented or dyed resin-based electroscopic particles, known as toner, and a granular carrier material which functions to carry such toner by generating triboelectric charges thereon. Upon contacting the electrostatic image-bearing surface, the toner particles, being charged to a polarity opposite to that of the electrostatic image, are separated from the carrier particles and are selectively deposited on the surface to form a toned image which may thereafter be transferred to a paper receiving sheet and fixed thereto by any suitable means, such as heat, to form a copy of the original document.
As toned images are repetitively formed, toner particles are depleted from the developer mixture, requiring subsequent replenishment to avoid a gradual reduction in image density. A variety of devices have been heretofore proposed for automatically replenishing toner particles after a predetermined number of copies are made, or alternatively, after a drop in the concentration of toner particles in the developer mixture below a predetermined level has been detected.
One device for detecting such a drop in toner concentration, which is disclosed in U.S. Pat. No. 3,233,781, issued to W. J. Grubbs, utilizes the difference in reflectivity exhibited by toner and carrier particles as means for monitoring the concentration of toner particles in the developer mixture. However, such optical devices have not proven entirely satisfactory in operation, especially over extended periods of time. A principal cause of unsatisfactory performance is the collection of airborne toner particles on the external surface of the various components of the monitoring apparatus, including the lamp, thereby gradually reducing the quantity of light received by the light sensing means. The low level of received energy causes the sensor to produce a false signal indicating a different concentration of toner than is actually present in the mixture. To assure freedom from erroneous measurements of this type, such monitoring apparatus requires frequent cleaning maintenance. Other causes of difficulty can be attributed to the instabilities, low output, and short life of the incandescent lamp used for illuminating the developer mixture. Another source of error in such apparatus is the random electrical perturbations, commonly called "noise," which are received and generated by the light sensing means.
Employment of the electrical properties of a developer mixture to monitor the suitability of the toner concentration overcomes the inherent deficiencies encountered in optical surveillance. For example, the dielectric constants for toner and carrier particles generally are not equal. Therefore, a change in the toner concentration in the mixture causes associated changes in the effective dielectric constant thereof. Toner concentration monitors which make use of the effective dielectric constant have been proposed. See, for example, U.S. Pat. No. 3,926,337, wherein a pair of electrically separated electrodes establish a field occupied by a developer mixture. Variations in the toner concentration within the developer mixture are indicated, in response to variations in the dielectric constant of the mixture, by an impedance bridge or Q-meter. While such indicating devices may be adequate for certain application, sensitivity is not high enough to provide a useful signal range for small changes in capacitance.